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Introduction to Molecular

SpectroscopyBy: M.Z.Iqbal

Lecture # 3 Physical Propertiesof Polymers


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Today

Introduction to spectroscopy

Classification of spectroscopic techniques

Common types and their introduction

Infra-Red spectroscopy: Fundamental concepts

Vibrations in IR- Region and calculations of differentvibrational energies in terms of frequencie.


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SpectroscopyDefinition:

The study of the interaction between radiations and matter as a function ofwavelength . Interactions with particle radiation or a response of a material to an alternatingfield or varying frequency .

Spectrum:A plot of the response as a function of wavelength or more commonly

frequency is referred to as a spectrum.Spectrometry:It is the measurement of these responses and an instrument which performssuch measurements is a spectrometer or spectrograph , although these termsare more limited in use to the original field of optics from which the conceptsprang.

Spectroscopy is often used in physical and analytical chemistry for theidentification of substances through the spectrum emitted from or absorbed bythem. Spectroscopy is also heavily used in astronomy and remote sensing. Mostlarge telescopes have spectrometers, which are used either to measure thechemical composition and physical properties of astronomical objects or tomeasure their velocities from the Doppler Shift of their spectral lines.


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Classification of MethodsThe type of spectroscopy depends on the physical quantity measured. Normally, thequantity that is measured is an intensity, either of energy absorbed or produced.

Most spectroscopic methods are differentiated as either atomic or molecular based onwhether or not they apply to atoms or molecules. Along with that distinction, they canbe classified on the nature of their interaction:

Absorption spectroscopy It uses the range of the electromagnetic spectra in which a substance absorbs. Thisincludes atomic absorption spectroscopy and various molecular techniques, such as

infra-red spectroscopy in that region and Nuclear Magnetic resonance spectroscopy inthe radio region.Emission spectroscopy

It uses the range of electromagnetic spectra in which a substance radiates (emits). Thesubstance first must absorb energy. This energy can be from a variety of sources, whichdetermines the name of the subsequent emission, like luminescence. Molecularluminescence techniques include spectroflourimetry.

Scattering spectroscopy It measures the amount of light that a substance scatters at certain wavelengths,incident angles, and polarization angles. The scattering process is much faster than theabsorption/emission process. One of the most useful applications of light scatteringspectroscopy is Raman Spectroscopy.
http://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Emission_spectroscopyhttp://en.wikipedia.org/w/index.php?title=Scattering_spectroscopy&action=edithttp://en.wikipedia.org/w/index.php?title=Scattering_spectroscopy&action=edithttp://en.wikipedia.org/wiki/Emission_spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopy


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Common types Fluorescence spectroscopy X-ray spectroscopy and crystallography Flame spectroscopy

1- Atomic emission spectroscopy2- Atomic absorption spectroscopy3- Atomic fluorescence spectroscopy

Plasma emission spectroscopy Spark or arc emission spectroscopy UV/VIS spectroscopy IR spectroscopy Raman spectroscopy

NMR spectroscopy Photo thermal spectroscopy Thermal infra-red spectroscopy Mass Spectroscopy


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Brief about spectroscopy

1- Fluorescence spectroscopy :Fluorescence spectroscopy uses higher energy photons to excite a sample,which will then emit lower energy photons. This technique has becomepopular for its biochemical and medical applications.


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Contd 2- X-ray spectroscopy:

X-rays of sufficient frequencies interact with material and excite the atomscontained. Due to this excitation Auger Effect is produced and someexcitation radiations are absorbed or evolved if vice versa occurs.X-ray absorption and emission spectroscopy is used in chemistry andmaterial sciences to determine elemental composition and chemicalbonding.

Very good and versatile technique but a little complex. It needs somescattering light detectors along with X-ray source. Overall X-ray diffractiontechnique is one that is used most widely for bond length and anglemeasurements.


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Contd 3- Flame Spectroscopy :

Liquid solution samples are aspirated into a burner or nebulizer/burnercombination, desolvated, atomized, and sometimes excited to a higherenergy electronic state. The use of a flame during analysis requires fueland oxidant, typically in the form of gases. Common fuel gases used areacetylene (ethyne) or hydrogen . Common oxidant gases used are oxygen ,air, or nitrous oxide . These methods are often capable of analyzingmetallic element in the PPM, billion, or possibly lower concentrationranges. Light detectors are needed to detect light with the analysisinformation coming from the flame.

- Atomic absorption Spectroscopy- Atomic emission spectroscopy

- Atomic fluorescence spectroscopy

How would you differentiate in them?
http://en.wikipedia.org/wiki/Acetylenehttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Earth%27s_atmospherehttp://en.wikipedia.org/wiki/Nitrous_oxidehttp://en.wikipedia.org/wiki/Nitrous_oxidehttp://en.wikipedia.org/wiki/Earth%27s_atmospherehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Acetylene


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4- Spark or arc (emission) spectroscopy :It is used for the analysis of metallic elements in solid samples.For non-conductive materials, a sample is ground with graphite powder tomake it conductive.In traditional arc spectroscopy methods, Since the conditions producingthe arc emission typically are not controlled quantitatively, the analysis for

the elements is qualitative. Nowadays, the spark sources with controlleddischarges under an argon atmosphere allow that this method can beconsidered eminently quantitative, and its use is widely expandedworldwide through production control laboratories of foundries and steelmills.


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Contd

5- UV/VIS spectroscopy: It basically involves the spectroscopy of photons and spectrophotometery. It uses light in the visible and adjacent near ultraviolet (UV) and near

infrared (NIR) ranges. UV/Vis spectroscopy is routinely used in the quantitative determination of

solutions of transition metal ions and highly conjugated organic

compounds . For the quantitative measurements, Beer-Lambert law is followed. The Beer-Lambert Law is useful for characterizing many compounds but

does not hold as a universal relationship for the concentration andabsorption of all substances. A 2nd order polynomial relationship between

absorption and concentration is sometimes encountered for very large,complex molecules such as organic dyes.
http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Quantitative_analysishttp://en.wikipedia.org/wiki/Transition_metalhttp://en.wikipedia.org/wiki/Conjugated_systemhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Conjugated_systemhttp://en.wikipedia.org/wiki/Transition_metalhttp://en.wikipedia.org/wiki/Quantitative_analysishttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Ultraviolet


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Contd

6- Infra-red Spectroscopy: (IR spectroscopy) is the subset of spectroscopy that deals with the

infrared region of the electromagnetic spectrum . It covers a range oftechniques, the most common being a form of absorption spectroscopy .As with all spectroscopic techniques , it can be used to identify compoundsor investigate sample composition.

Infrared spectroscopy offers the possibility to measure differenttypes of inter atomic bond vibrations at different frequencies. Especially inorganic chemistry the analysis of IR absorption spectra shows what type ofbonds are present in the sample.

Infrared spectroscopy exploits the fact that molecules have specificfrequencies at which they rotate or vibrate corresponding to discreteenergy levels.
http://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Organic_chemistryhttp://en.wikipedia.org/wiki/Organic_chemistryhttp://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Spectroscopy


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Contd

7- Raman Spectroscopy: It relies on inelastic scattering, or Raman scattering of

monochromatic light, usually from a laser in the visible , near infrared , ornear ultraviolet range. The laser light interacts with phonons or otherexcitations in the system, resulting in the energy of the laser photonsbeing shifted up or down. The shift in energy gives information about thephonon modes in the system.

Spontaneous Raman scattering is typically very weak, and as a resultthe main difficulty of Raman spectroscopy is separating the weakinelastically scattered light from the intense Rayleigh scattered laser light.Raman spectrometers typically use holographic diffraction gratings andmultiple dispersion stages to achieve a high degree of laser rejection.
http://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Visiblehttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Phononhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Spectrometerhttp://en.wikipedia.org/wiki/Diffraction_gratinghttp://en.wikipedia.org/wiki/Diffraction_gratinghttp://en.wikipedia.org/wiki/Spectrometerhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Phononhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Visiblehttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Raman_scatteringhttp://en.wikipedia.org/wiki/Elastic_scattering


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Contd

8- Nuclear magnetic resonance:Nuclear magnetic resonance spectroscopy analyzes the magnetic

properties of certain atomic nuclei to determine different electronic localenvironments of hydrogen , carbon , or other atoms in an organiccompound or other compound . This is used to help determine thestructure of the compound.

9- Photo thermal spectroscopy:It is a group of high sensitivity spectroscopy techniques used to measureoptical absorption and thermal characteristics of a sample. The basis ofphoto-thermal spectroscopy is the change in thermal state of the sampleresulting from the absorption of radiation. Light absorbed and not lost byemission results in heating. The heat raises temperature thereby

influencing the sample thermodynamic properties. Measurement of thetemperature, pressure, and/or density changes that occur due to opticalabsorption are ultimately the basis for the photo-thermal spectroscopicmeasurements.
http://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_structurehttp://en.wikipedia.org/wiki/Chemical_structurehttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Hydrogen


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Contd

10- Thermal infrared spectroscopy :TIR spectroscopy is the subset of IR spectroscopy that deals with radiation

emitted in the infrared part of the e.m. spectrum. The emitted infraredradiation, though similar to blackbody radiation, is different in that theradiation is banded at characteristic vibrations in the material. Themethod measures the thermal IR radiation emitted (as opposed to beingtransmitted or reflected) from a volume or surface. This method iscommonly used to identify the composition of surface by analyzing itsspectrum and comparing it to previously measured materials. It isparticularly suited to airborne and space borne applications.


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Infra Red SpectroscopyIntroduction:

The absorption of infra-red radiations causes the various bands in a

molecule to stretch or bend w.r.t. one another.

Near IR-Region 0.8 2.5 micron Rotational Spec.Mid infra red region 2.5 1.5 micron Vib-Roto Spec.Far IR-Region 15 200 micron Overtone or harmonic

Band intensity is either expressed as absorbance or transmittance.


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Contd

To establish the identity of two compounds (comparative study)

To determine the structure of a new compound from its functional groups

To determine the nature of contaminants in a sample

For quantitative analysis of a component in the overall mixture

For the quantitative analysis of contaminants in given sample

Some advanced physical properties of the materials


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Basic Principles: Molecular Vibrations Absorption in the infra-red region is due to the changes in the vibrational

and Rotational levels. With low energy radiations, molecular rotations occur With high energy radiations, molecular vibrations occur

Due to IR-radiation energy, a vib-rotational spectrum is obtained. Thevibrational energy depends upon the followings:1- masses of atoms present in the molecule2- Strength of bonds and bond distances3- The arrangement of atoms within a molecule


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Types of vibrations in a molecule

Two fundaments types:1- Stretching2- Bending

Stretching:The distance between the two atoms increases or decreases but the

atoms remain in the same bond axis.

Bending:The position of the atoms changes with respect to the original bond axis.

Stretching occurs at high frequencies and bending at low ?


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Contd

Stretching Vibrations:

a) Symmetric stretchb) Asymmetric stretch

Bending Vibrations:a) Scissoringb) Rockingc) Waggingd) Twisting

See the moving image


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Vibrational Frequency Stretching vibrational frequency of a bond can be calculated through

Hooks Law as:

Wherem1,m2 are masses of bonded atomsK is force constant of the bond depends upon the strength of bondK = 5 x 10 ^5 g/cm^2 for single bondsC = Velocity of radiation = 2.99 X 10^10 cm/secV = Frequency of radiationV = Wave number

Vib-frequency depends upon bond strength and reduced mass


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Do yourselfCalculate the bond vibrating frequencies and the wave numbers

for the followings:1. C C 11. O = O

2. C O 12. C = S3. H H 13. C = P

4. O H 14. C = Si5. O O 15. C =_ C

6. O F 16. C Cl7. C = C 17. C - Al

8. C = O9. C = N

10. C = Fe Extract the conclusions?


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